A known gastric fistula catheter or trocar in the related art is equipped, at an distal end thereof, with a balloon or a superelastic expanding member that is expandable in a radial direction and that is designed to prevent the catheter or trocar from coming out by expanding the balloon or the superelastic expanding member, in a state in which it penetrates a tissue wall so that the distal end is inserted in a body (for example, see PTLs 1 and 2).